1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a liquid crystal display device having a thin film transistor (TFT) and a method of manufacturing the same.
2. Description of Related Art
A typical liquid crystal display device uses optical anisotropy and polarization properties of liquid crystal molecules. The liquid crystal molecules have a definite orientational order in arrangement resulting from their thin and long shapes. The arrangement direction of the liquid crystal molecules can be controlled by supplying an electric field to the liquid crystal molecules. In other words, as the arrangement direction of the liquid crystal molecules is changed, the arrangement of the liquid crystal molecules also changes. Since Incident light is refracted to the arrangement direction of the liquid crystal molecules due to the optical anisotropy of the arranged liquid crystal molecules image data can be displayed.
By now, an active matrix LCD that the thin film transistors and the pixel electrodes are arranged in the form of a matrix is most attention-getting due to its high resolution and superiority in displaying moving video data.
FIG. 1 is a cross-sectional view illustrating a pixel of a conventional liquid crystal display device.
The liquid crystal display device 20 has a bottom substrate 2 and a top substrate 4 spaced apart from each other. The liquid crystal display device further includes a liquid crystal layer 10 is injected between the two opposite substrates 2 and 4. The top substrate 4 has a color filter to display colors, and the bottom substrate 2 has switching elements such as thin film transistors (TFTs) that applies electrical signals to the liquid crystal layer 10 to change the arrangement direction of the liquid crystal molecules of the liquid crystal layer 10. Each of the TFTs “S” has a gate electrode 30, a source electrode 32 and a drain electrode 34.
In detail, the top substrate 4 further includes a common electrode 12 covering the color filter layer 8. The common electrode 12 plays a role of the first electrode to supply a voltage to the liquid crystal layer 10. The bottom substrate 2 further includes a pixel electrode 14. The pixel electrode 14 is electrically connected with the drain electrode 34 of the TFT “S”. The pixel electrode 14 receives electrical signals from the thin film transistor “S”, and plays a role of the second electrode to supply voltage to the liquid crystal layer 10. A portion, on which the pixel electrodes 14 are formed, is defined as a pixel electrode portion “P”. In order to prevent leakage of the liquid crystal layer 10 disposed between the top substrate 4 and the bottom substrate 2, edge portions of the top substrate 4 and the bottom substrate 2 are sealed by a sealant 6.
Recently, as the display area of the liquid crystal display device becomes larger, the fabricating process of the bottom substrate 2 becomes complicated. That is to say, for a liquid crystal display device having over 12 inch display area, a step-and-repeat exposure technique is applied to fabricating the bottom substrate. The step-and-repeat exposure technique is to perform at least two exposing steps with the same patterned mask. The reason for the step and repeat technique to be applicable to the bottom substrate is that the patterns formed on the bottom substrate are repeats of the same form.
Referring to the FIGS. 2 and 3, a batch exposure technique and the step-and-repeat exposure technique are explained as follows. Referring to the FIG. 2 showing a patterning mask, in the batch exposure technique, a display area A, data pad portions D and E and gate pad portions B and C surrounding the display area “A” are formed at one time with the patterning mask.
The batch exposure technique is just applicable to the bottom substrate of the liquid crystal display device having a less than 10 inch-sized display area. Namely, in case of the bottom substrate of the liquid crystal display device having a larger than 10 inch-sized display area, the batch exposure technique is useless due to the diffraction of light incident from an exposure apparatus.
Referring to the FIG. 3 illustrating the step-and-repeat exposure technique, the display area is formed into a plurality of neighboring display exposure regions like A1, A2, . . . , A9, sequentially. Each of the display exposure regions has an identical image projected onto itself with a same display patterning mask.
By the same technique, the data pad portions are formed into a plurality of neighboring data exposure regions like D1, D2, D3 and E1, E2 E3 having an identical image, sequentially. And the gate pad portions are formed into a plurality of neighboring gate exposure regions like B1, B2, B3, C1, C2 and C3 having an identical image, sequentially.
The above-mentioned step-and-repeat exposure technique is more widely used than the batch exposure technique as an exposure method.
But, to fabricate the liquid crystal display device using the step-and-repeat exposure technique may give rise to a serious degradation of image quality at the display area. The reason is that the step-and-repeat exposure technique needs at least over 40 processes of photolithography. Comparing with the step-and-repeat exposure technique, the batch technique needs at least just 5 processes of photolithography. Thus, no matter bow accurate exposure equipment and arrangement apparatus are used for the step-and-repeat exposure technique, it may give rise to misalignment between the exposure regions.
For example, as shown in FIG. 4, the display exposure regions A7 and A8, the display exposure region A7 includes a pixel electrode 71, a half of a data line 60, and a half of a data line 61. The display exposure region A8 includes a pixel electrode 72, a half of a data line 61, and a half of a data line 62. The display exposure regions A7 and A8 include the data line 61 in common, and are divided by an imaginary boundary line 50. That is to say, the display exposure regions A7 and A8 differ in an exposed order with the imaginary boundary line 50 centering on between the display exposure regions A7 and A8. That difference in the exposed order may bring out a difference in distances between the pixel electrodes 71 and 72 and the data lines 60, 61 and 62. Since the exposure equipment or the arrangement apparatus has an accuracy limitation of itself, misalignment between the exposure regions may occur. The misalignment may result in shift, rotation and distortion of the patterns, thereby causing defects such as disconnection of the wirings and differences in electrical properties between the exposure regions.
Namely, the distance between the pixel electrode 71 and the data line 60 is different from the distance between the pixel electrode 71 and the data line 61. And, the distance between the pixel electrode 72 and the data line 61 is different from the distance between the pixel electrode 72 and the data line 62. The pixel electrodes 71 and 72 are the pixel portions P1 and P2, respectively.
In other words, fabricating the thin film transistor by the step-and-repeat exposure technique, it may bring about spotted effects near the boundaries of the neighboring display exposure regions resulted from the sudden difference in the distance between the pixel electrodes and the data lines at each exposure region.
In case of manufacturing the large-sized liquid crystal display device using the step-and-repeat exposure technique, driving the liquid crystal display device by a dot inversion method, it brings about the difference in parasitic capacitance Cdp between the data line and the right and left pixel electrodes between exposure regions. The parasitic capacitance Cdp is the critical factor directly affecting the brightness of the display area. Thus, the difference in the parasitic capacitance Cdp brings about the difference in the brightness between the left pixel electrode and the right pixel electrode with the center boundary line differentiating the left pixel electrode and the right pixel electrode. Namely, the detectable difference in brightness, or the spotted effect occurs near the boundaries of the exposure regions.